Our overall hypothesis is that the sudden infant death syndrome (SIDS), or a subset of SIDS, is due to developmental abnormalities of the ventral medulla that interfere with normal protective cardiorespiratory responses to potentially life-threatening, but often occurring, events during sleep, such as hypoxia, hypercapnia, and apnea. We recently reported in SIDS victims neurotransmitter receptor binding deficiencies in the arcuate nucleus, which contains ventral medullary surface neurons that are considered homologous to neurons located in similar areas in cats that are necessary for the protective responses to hypercapnia and asphyxia. We propose a triple-risk model of SIDS in which an infant will die of SIDS only if he/she possesses: 1) an underlying vulnerability, e.g., an abnormality in the ventral medulla; 2) a critical period in the development of homeostatic control, i.e., early infancy; and 3) an exogenous stressor, e.g., positional asphyxia. This model will be examined in Projects I-III, both in decerebrate piglets and in chronically instrumented piglets during wakefulness and natural sleep. In Project I, we will define the role of the arcuate homologue in cardiorespiratory control in the piglet across early development, and determine the effect of structural and neurochemical lesions of the arcuate homologue on response to hypercapnia and asphyxia. In Project II, we will delineate in piglets the mechanisms of protective ventilatory response to hypoxia, especially the depressive phase of the biphasic response and autoresuscitation by gasping. In Project III, we will determine the effects of trigeminal, superior laryngeal, and intercostal nerve stimulation in piglets with arcuate homologue lesions during room air exposure, hypercapnia, and hypoxia. In Project IV, we will define the comparative cyto-and chemoarcitectonic anatomy of the human arcuate nucleus and the piglet homologue. In Project V, the objective is to apply information from Projects I-IV in the definition of abnormalities in SIDS brainstems, in addition to those in the arcuate nucleus. The A. Administrative Core will provide the necessary administrative framework. The B. Anatomy Core will provide cellular and neurochemical services for animal and human brainstem studies. The C. Animal Physiology Core will provide an animal research laboratory in which Projects I-IV will be performed by the individual investigators. This program should advance our understanding of the role of ventral medullary abnormalities in the pathogenesis of SIDS.